It's a continuation of a growing pain with many applications and APIs still not dealing with sensor orientation properly. Everything in the stack from the camera, to the photo or video viewer app and any image/video editors must support the orientation flag and handle it correctly. The way the Nexus 5X sensor is oriented is within acceptable standards. The Reddit comment from the Google engineer indicates the older camera API Android used put the requirement of handling rotation more on the application. Due to most Android phones using a different sensor orientation, most app developers didn't notice the requirement as they didn't come across an issue during development. The default worked more due to luck of most every phone using the same orientation. Odds are some lesser known Android phones have had similar over the years, the Nexus 5X has a high enough profile in the ecosystem to perhaps move towards more apps fixing this issue.

For your particular situation, that stack now involves whatever your using to access the videos on the NAS as well. I remember a burst of stories around the time the iPhone gained video support about upside down videos due to Windows Media Player of the era supporting the Quicktime contained MP4 video, but not supporting the orientation tag properly.

http://www.impulseadventure.com/photo/exif-orientation.html talks about this from the point of view of a photo. EXIF is where orientation data is stored. With video, it's also still metadata, but a bit more deeply embedded as part of how video works. A much higher level discussion including vector math explaining the metadata is here: http://stackoverflow.com/questions/7025186/how-to-remove-or-edit-exif-from-mp4-video . There's a chance this isn't as standardized with video as it is images via EXIF, I didn't research to see if this method is unique to MPEG 4 or a more general standard seen across MPEG/Microsoft/VP video.